Toxoplasma bacterium is a common feline parasite.  It normally is shed in cat feces that are then ingested by rats.  The bacteria make the rats fearless in the presence of cats, and that makes them easier to catch and consume.  When the cat ingests the infected rat, it becomes infected with Toxoplasma.  By altering the personality of the rat so that it sacrifices itself to the intended predators, the bacteria are able to spread to more feline hosts and continue their lifecycle.

This behavior is not surprising as parasites use a great many tricks in order to spread.  What is surprising is how Toxoplasma seems to alter human behavior.

It was long thought the bacteria had very little affect on people since it is not unusual for humans to be infected.  To most, such an infestation was largely harmless, although those with immunosuppressive conditions were at risk of further compromise and related infections.

Parasitologists released new information from the study of Toxoplasma bacteria in people, information that shows the original assumptions about its impact may have been incorrect.  Like the personality change it causes in rats in order to make them easier prey for cats, human infections also indicate personality changes.  Most important of note in these changes is that they are different between men and women.

Some scientists believe that Toxoplasma changes the personality of its human hosts, bringing different shifts to men and women. Parasitologist Jaroslav Flegr of Charles University in Prague administered psychological questionnaires to people infected with Toxoplasma and controls. Those infected, he found, show a small, but statistically significant, tendency to be more self-reproaching and insecure. Paradoxically, infected women, on average, tend to be more outgoing and warmhearted than controls, while infected men tend to be more jealous and suspicious.

That’s right.  While no smoking gun exists right now, there are clear indications that Toxoplasma infections cause opposite personality changes in men and women in addition to those common with all infected people.  While females become friendlier, men essentially become jerks, and both changes seem to work in exactly the same way that makes rats fearless of cats.

Further studies also show the bacteria may play a causal role in the development of schizophrenia.

Infection with the parasite has been associated with damage to a certain class of neurons (astrocytes). So has schizophrenia. Pregnant women with high levels of Toxoplasma antibodies in their blood were more likely to give birth to children who would later develop schizophrenia. Torrey lays out more links in this 2003 paper. While none is a smoking gun, they are certainly food for thought. It’s conceivable that exposure to Toxoplasma causes subtle changes in most people’s personality, but in a small minority, it has more devastating effects.

A year later, Torrey and his colleagues discovered one more fascinating link. They raised human cells in Petri dishes and infected them with Toxoplasma. Then they dosed the cells with a variety of drugs used to treat schizophrenia. Several of the drugs–most notably haloperidol–blocked the growth of the parasite.

This begged the question of whether schizophrenia treatment drugs had a similar impact on infected rats, the most notable host with demonstrable psychological changes caused by the bacteria.

So Fuller and the Oxford scientists joined forces to find an answer to the next logical question: can drugs used to treat schizophrenia help a parasite-crazed rat? They now report their results in the Proceedings of the Royal Society of London (press release). They ran the original tests on 49 more rats. Once again, parasitized rats lost their healthy fear of cats. Then the researchers treated the rats with haloperidol and several other anti-psychotic drugs. They found that the drugs made the rats more scared. They also found that the antipsychotics were as effective as pyrimethamine, a drug that is specifically used to eliminate Toxoplasma.

All of this is of extreme interest.  Knowing Toxoplasma causes personality changes in rats, thereby making them easier prey for cats — the intended host, begs the question of whether the same is true of infected humans.  Nothing definitive is available on that yet, but the evidence to date appears indicative and provides ample reason to continue its study.  Possible differences in human impact as determined by gender are equally impressive and worthy of note, especially as there are known psychological variations based on human sexual biology.  Finally, the continuing study could hold the key to better treatment (and prevention?) of schizophrenia (or flavors thereof).

And all from one very small bacterium.